The present invention relates to paper manufacturing and papermaking machinery and methods, and in particular, to a process of drying a fibrous web in a single-felt dryer group wherein low vacuum is provided to the vacuum transfer rolls between dryer cylinders.
WO 83/00514 discloses the principle of drying a fibrous web in a single-felt dryer group having drying cylinders and suction rolls, without giving details on the degree of negative pressure in the suction rolls.
An article entitled Advances in Dryer Section Runnability, by G. L. Wedel & S. Palazzolo, TAPPI Journal, September 1987, pp. 65-69, discloses at p.67 "high vacuum" of 1000 Pa (approx. 4 in. H.sub.2 O) to hold the web to the "fabric" or porous support felt or belt, without giving details of the influence of the negative pressure upon shrinkage of the web.
U.S. Pat. No. 5,279,049 requires 1490-1990 Pa (approx. 6-8 inches H.sub.2 O) in the suction rolls in order to inhibit cross-directional web shrinkage "in the dry end of the dryer section".
U.S. Pat. No. 5,241,760 states that suction rolls are "unnecessary" in certain applications. Suggested are grooved transfer rolls without suction, with "inherent machine directional shrinkage of the web during drying thereof" inhibiting cross-directional web shrinkage.
DE 4328554A1 shows various dryer sections wherein the present invention is applicable.
The information contained in the second, third and fourth references above appears to be incomplete or at least ambiguous.
Also, it was found that the very high negative pressure required by U.S. Pat. No. 5,279,049 tends detrimentally to affect the web during the drying process. The result is low quality of the finished web. Further, in many cases, wrinkles are formed in the web, namely, in the web path around the suction rolls so that the web is unsaleable, resulting in a loss of productivity of the paper-making machine.
In certain applications (depending on the grade and/or on the basis weight of the fibrous web) even 1000 Pa (approx. 4 in. H.sub.2 O) may be too high in order to achieve a finished web having the quality and the properties required (e.g., for printing purposes).